Mechanical pencils are known in which the lead is protected against breaking by means of a tubular lead guide which projects beyond one end of the tubular casing of the pencil and which surrounds the lead substantially up to the writing point thereof. Such pencils are especially adapted for use with thin and/or soft leads, for instance with leads which have a diameter of 0.5 or 0.3 mm, and which are eventually also rather soft. Such pencils with thin leads make it possible to draw or write with fine or heavy lines, which is possible with mechanical pencils using relatively large diameter leads and with wooden pencils, only if the leads are carefully sharpened. Such repeated careful sharpening of the lead is, however, avoided with thin-lead mechanical pencils.
A disadvantage of mechanical pencils of the aforementioned kind is that the position of the lead relative to the lead guide has to be repeatedly and exactly adjusted since the lead will not write any longer when its end projecting beyond the lead guide is used up, and since if the lead extends too far beyond the lead guide it will easily break, and that therefore the adjustment of the position of the lead relative to the lead guide must be made much more carefully than with mechanical pencils of the usual heavy lead type.
An attempt has already been made to avoid the aforementioned disadvantage by a construction in which the lead guide is not fixedly connected to the casing, but is mounted in the lower end of the casing for movement in longitudinal direction, and wherein the lead guide engages the lead with a predetermined frictional force which is chosen in such a manner that the lead will be moved relative to the lead guide by a small pressure, for instance by engagement of the lead guide with the writing surface or by an advancing movement of the lead.
During writing or drawing, the lead will be pushed back by the pressure exerted thereon so that practically only the end face of the lead, that is the acting writing face thereof will project beyond the tubular lead guide. As the lead is used up, the lead guide will come into contact with the writing surface and will thereby gradually be pushed back into the pencil casing, so that only the end face of the lead will remain in contact with the writing surface, whereas the portion thereof projecting beyond the pencil casing will be surrounded and thereby protected by the non-bendable tubular lead guide.
Thus the necessity of repeatedly adjusting the lead is avoided, and the position of the lead has to be adjusted only when its whole length projecting beyond the lower end of the casing is used up.
It is also known in the art of mechanical pencils to adjust the position of the lead automatically as a function of pressure exerted on the tubular lead guide. However, such pencils are constructed of a large number of parts, each of which perform a separate function, thereby making the entire assembly complex and expensive. Thus, the prior art provides one member for clamping the lead during writing, another member for transmitting axial pressure from the tubular lead guide member to the clamping member, and yet another member for restoring the tubular lead guide member back to its original position when the axial pressure has been terminated. This multi-element construction makes the cost and construction of such pencils very prohibitive.